What is the probability of choosing a diamond card out a regular deck of 52?

There are 13 diamonds and each has the same probability of being chosen, namely, 1/13. 3 of the diamonds are face cards. Therefore, the probability of choosing a face card given that it's a diamond is 3/13.

There are 13 diamonds in a regular deck of cards.

So the probability of picking a diamond card is 13/52.